This invention pertains to filtration media composites, and pertains particularly to gas filtration media composites formed of a electret media layer attached uniformly to a second media layer by controlled needling so as to provide media composites having enhanced performance for gas filtration systems.
Layered filtration media for removing fine solid particulate matter from gases in which it is suspended are generally known, and may include either a layer of blended electrostatic fibers on an electret layer material and a conventional non-woven fibrous layer. For example, U.S. Pat. No. 2,724,457 to Besser discloses an electrostatic air filter utilizing loosely woven electrostatic synthetic plastic yams so woven as to form irregular projections and pockets which disturb air flow to cause precipitation of dust particles in the filter. U.S. Pat. No. 2,916,038 to Wade discloses a tobacco smoke filter formed either of organic fiber electrets or a mixture of such electret fibers with non-electret fibers, such as natural or synthetic fibers of non-uniform diameter. The fibers may be rolled together and also twisted to form a cylindrical-shaped mass which can be bonded together by use of a binder or by heat.
U.S. Pat. No. 4,257,791 to Wald discloses a filtering medium formed of non-woven needled textile material having a bulk density gradient such that the front or facing portion has a greater bulk density than the rear portion of the filter U.S. Pat. No. 4,798,850 to Brown, discloses a blended fibrous filter material in which two types of fibers each carrying opposite electric charges are needled together so as to attract and retain dust particles and provide high filtration efficiency.
U.S. Pat. No. 4,880,448 to Scherrer discloses a double layer filter insert for an air cleaning apparatus, including a first pre-filter of polyester fibers as an upper or outer layer facing the inflow of dust laden air, and a second layer of an electrostatic nonwoven fabric loosely disposed behind as a lower or inner layer. The two layers are wedded together along their longitudinal edges. U.S. Pat. No. 4,917,942 to Winters discloses a fibrous filtration laminate including a self-supporting non-woven fabric and an intertangled non-woven mat of electret containing microfibers deposited on and adhering to the first layer.
U.S. Pat. No. 5,112,677 to Tani et al discloses an electret sheet construction including a porous sheet made of a dielectric polymer and at least one solid material selected from organic, inorganic materials, and metallic materials, and includes material spaced at various intervals on the surface of the dielectric polymer. Also, U.S. Pat. No. Re. 35,236 to Nolan discloses an air filter construction employing layers of woven media within a frame, increasing two layers of electrostatic polypropylene woven fabric in an "egg-crate" pattern, each separated by a centrally disposed polyester pad, and held together through a frame device.
Although these prior filtration media constructions have been found useful, further improvements are desirable to provide filtration media having increased durability and filtration efficiency for fine particle removal from flowing gas streams.